Field of Invention
The present invention relates to field of detecting technique, and in particularly to a micro-magnetic detecting method and a micro-magnetic detecting device for detecting an underground object so as to inspect the external surface and internal defect of the detected object.
Description of Related Arts
With the advance of inspection technology especially for the industrial production, a nondestructive inspection technology is a need for more and more target objects, such as a long distance pipeline. For example, a long distanced oil and gas pipeline is usually buried underground. A regular work flow for detecting such buried pipeline defect includes the steps of excavating areas of the pipeline site, removing an anticorrosion (heat preservation) layer, inspecting the pipeline, enveloping back the pipeline, and backfilling the pipeline. Such work flow will certainly affect the normal operation of the buried pipeline. Obviously, different pipeline inspection methods for the buried pipeline without excavation or shutting down the pipeline have become an issue that is worth further discussion.
Apparently, a nondestructive inspection or detecting is needed for the target objects like buried pipeline. That is, with a prerequisite of not letting the target object being damaged, separated, or broken, the inspection technique is incorporated with physical methods, including sound, light, electricity, magnetism, etc., to detect properties, conditions, and/or internal structures of a material, a product, or a structure. Here, detecting methods or techniques that are widely utilized in detecting internal and surface defects of the target object are also called nondestructive detection.
There are many nondestructive detecting methods in the art, including ultrasonic detecting method, turbulent flow detecting method, ray detecting method, etc. . . The ultrasonic detecting method uses an interaction between ultrasonic wave and the target object to provide feedback to be implemented. Although sound can be transmitted in metals, ultrasonic wave decays quickly in the air, which is the major drawback of this method. As a result, it usually requires coupling agent, such as oil or water, serving as a communication medium for the sound.
Ray detecting method is also a nondestructive detecting method that utilizes physical effect (such as a change of radiation intensity, scattering, etc.) generated in an interaction between ionization radiation and the object, so as to detect a discontinuity, a structure, or a thickness inside of a workpiece.
Turbulent flow detecting method is based on electromagnetic induction theory, so it can detect surface defects and near-surface defects of the workpiece. Outstanding features of the turbulent flow detecting method are that it works on conductive materials, which are not necessarily ferromagnetic materials. However, it functions poorly on ferromagnetic materials. Besides, smoothness, flatness, and boundary of surfaces of the workpiece to be detected will all greatly influence the turbulent flow. Therefore, the turbulent flow detecting method is usually applied to detect flaw on non-ferromagnetic workpiece with relative regular shape and smooth surface, such as copper pipe. Moreover, the turbulent flow detecting method also requires exciting source, which becomes not applicable when there is no exciting source in certain circumstances.
Therefore, there needs improved nondestructive detecting techniques.